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A novel approach for the synthesis of monodispersed porous silica microspheres with high surface area
Journal of Non-Crystalline Solids 351 (2005) 3593–3599 www.elsevier.com/locate/jnoncrysol
A novel approach for the synthesis of monodispersed porous silica microspheres with high surface area Li Zhao a, Jiaguo Yu a
a,*
, Bei Cheng a, Chengzhong Yu
b
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Luoshi Road 122#, Wuhan 430070, PR China b Department of Chemistry, Fudan University, Shanghai 200433, PR China Received 1 February 2005; received in revised form 23 September 2005 Available online 26 October 2005
Abstract Monodispersed porous silica microspheres are synthesized by the hydrolysis and condensation of tetraethoxysilane (TEOS) in a water–ethanol mixed solution containing 1-alkylamine as a template and hydrolysis catalyst. The as-prepared products were characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM), small angle X-ray diffraction (SAXRD), Fourier transform infrared spectroscopy (FTIR) and nitrogen adsorption, respectively. It was found that the alkyl chain length of 1-alkylamine and calcination temperature have an obvious influence on the particle size, morphology, specific surface area and pore structure of the as-prepared silica powder. The specific surface area, porosity and pore volume increased with increasing calcination temperature. Further observation showed that at 600 °C, with increasing the alkyl chain length of template from C12 to C18, the specific surface area decreased and the pore size, porosity and pore volume increased. This research may provide new insight into the control of morphology and pore structures of oxide materials. Ó 2005 Elsevier B.V. All rights reserved. PACS: 61.43.Er; 61.43.Gt; 81.20.Fw
1. Introduction The development of mesoporous silica materials by researchers of Mobil Corporation in 1992 stimulated explosive research on the preparation of porous materials through template approaches [1,2]. That was due to the fact that the porous materials had their potential applications as versatile catalyst, catalyst supports, separation media, and hosts for clusters and nanowires [3–6]. Various types of organic templates including surfactant self-assemblies, block copolymer and polymer lattices have been used to synthesize such porous inorganic materials. Recently, the synthesis of monodispersed spherical particles by the
*
Corresponding author. E-mail address: [email protected] (J. Yu).
0022-3093/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.09.031
sol–gel method has focused more attention on the design of spherical silica-based materials with tailored pore structures and properties [7–10]. In this regard, significant effort has been devoted to the organic templating or molecular imprinting of porous silica systems in which the templates interact covalently or non-covalently with silica precursors [7,11,12]. For example, Vacassy et al. [13] used 3-aminopropyltriethoxy-silane and glycerol as porogens to prepare microporous silica spheres. Qi et al. [14] used double-hydrophilic block copolymers (PEO-b-PMMA) as template under strong acidic conditions to form microporous silica spheres. Ma et al. [15] used a triblock copolymer EO20PO70EO20 as template in combination with a co-surfactant CTAB to prepare silica microspheres via a two-step synthesis process. Lefe`vre et al. [16] reported that preparation of large-pore mesostructured micelle-templated silica spheres. Although a variety of porous silica spherical
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particles have been synthesized so far, almost all the silica spheres have been obtained by using a triphasic system of a quaternary ammonium surfactant with an additional auxiliary organic solvent and co-surfactant. In this study, porous silica microspheres with narrow particle size distribution were successfully prepared by hydrolysis and condensation of tetraethoxysilane (TEOS) in a mixed solution of water, 1-alkylamine and ethanol. The key in this experimental process is the dual role of 1-alkylamine, not only as a porous template but also as a basic catalyst for the hydrolysis and condensation of TEOS. To the best of our knowledge, this was a very simple and more effective method for producing monodispersed silica spherical particles with a very large specific surface area. The formation mechanism of porous silica microspheres with disordered pores was proposed based on these experimental results. 2. Experimental section 2.1. Preparation The porous silica microspheres were synthesized using 1-alkylamine (designated C12 for dodecylamine, C16 for hexadecylamine and C18 for octodecylamine) as a template and hydrolysis catalyst, and TEOS as an inorganic framework sources. In a typical preparation, 0.005 mol of 1-alkylamine was dissolved in a mixed solution of ethanol (160 ml) and distilled water (100 ml). Then, 10 ml TEOS was added dropwise to the above solution at about 15 °C. After 4 h, the white precipitate was filtrated. The products were repeatedly washed with water and ethanol for four times, then dried in a vacuum oven at 80 °C for 4 h, finally calcined at 400 and 600 °C for 4 h in a muffle furnace to remove the templates, respectively. 2.2. Characterization The morphology of the silica products was observed by scanning electron microscopy (SEM) (type JSM-5610LV) with an accelerating voltage of 20 kV. Transmission electron microscopy (TEM) study was carried out on a Philips TECNAL-10 electron microscopy instrument. The sample for TEM was prepared via milling and dispersing the final powders in ethanol. The dispersion was then sonicated and dropped onto carbon-coated copper grids. Small angle X-ray diffraction (XRD) patterns were recorded on an HZG41B-PC X-ray diffractometer using Cu-Ka radiation with a 2h range of 1–5°. Fourier transform infrared (FTIR) spectra on pellets of the samples mixed with KBr were recorded on a Nicolet Magna 8100 FTIR spectrometer. The Brunauer–Emmett–Teller (BET) surface area (SBET) and pore parameters of the silica products were analyzed by nitrogen adsorption–desorption isotherm measurements on an AUTOSORB-1 (Quantachrome Instruments, USA) nitrogen adsorption apparatus. For the samples dried at 80 °C, they were degassed at 70 °C prior to actual measure-
ments. However, for the samples calcined at high temperatures (from 400 to 600 °C), the degassing temperature was 180 °C. The BET surface area was determined by the multipoint BET method using the adsorption data in the relative pressure (p/p0) range of 0.05–0.25. The desorption branch of the nitrogen isotherm was used to determine the pore size distribution using Barret, Joyner, and Halender (BJH) method [17]. The porosity was calculated according to the following equations (the skeleton specific volume of SiO2 is taken as 0.37 cm3 g1) [18], P ¼ V p =ðV p þ 0:37Þ;
ð1Þ
3
V p ¼ 1:547 10 V d ;
ð2Þ
where Vp is the volume of the liquidated nitrogen corresponding to the total pore volume, which was calculated from the saturation adsorption volume at STP, Vd. 3. Results Fig. 1 shows the small angle XRD patterns of the assynthesized products using dodecylamine, hexadecylamine and octodecylamine as templates and calcined at 600 °C for 4 h. All XRD patterns are similar and exhibit a broad low-angle diffraction peak corresponding to d100 spacing of 3.2, 5.6, 7.1 nm for C12, C16 and C18, respectively. It is found that the d100 spacing of the silica samples increases with increasing the alkyl chain length of template. This is attributed to the pore size expansion as the alkyl chain length increased from C12 to C18. The absence of higher angle refraction peaks indicates that these particles lack the long-range order in the pore arrangement that was commonly seen for conventionally prepared highly ordered microporous and mesoporous silica materials. Fig. 2 shows the influence of different templates on the particle sizes and morphologies of SiO2 particles calcined at 80 and 600 °C. It can be seen that the size and morphology of SiO2 particles are obviously different for different templates. Furthermore, the morphologies of the as-syn-
(c) (b) (a)
Fig. 1. Small angle XRD patterns of porous silica microspheres obtained using dodecylamine (a), hexadecylamine (b), and octodecylamine (c) as templates and calcined at 600 °C for 4 h.
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Fig. 2. SEM images of silica microspheres obtained using dodecylamine as template and calcined at 80 °C (a) and 600 °C (b), hexadecylamine as template and calcined at 80 °C (c) and 600 °C (d), and octodecylamine as template and calcined at 80 °C (e) and 600 °C (f).
thesized particle are retained after the calcination treatments. This result shows that the calcination treatments cannot obviously affect the size and morphology of silica particles. For C12 powder, the SiO2 particles show uniform size and spherical morphology. The average diameter of the SiO2 microspheres is about 700 nm. For C16 powder, the SiO2 particles appear polydispersity. And the average size of the SiO2 particles obviously becomes large and is about 1.5 lm. In contrast, the C18 powders have a largest particle size and exhibit some plate-like aggregate particles in addition to some spherical particles. Therefore, the morphologies and sizes of SiO2 particles are strongly influenced by the alkyl chain length of 1-alkylamine. The particle size, monodispersity and morphology of SiO2 particles can be clearly observed in the TEM image. Fig. 3(a) shows TEM images of the monodispersed silica microspheres synthesized using dodecylamine as a template. As can be seen from HRTEM image (as shown in Fig. 3(b)), wormhole-like pore structures are randomly connected and lack discernible long-range order in the pore arrangement among the small SiO2 particles. Fig. 4(a) and (b) show the FTIR spectra of silica microspheres obtained using dodecylamine as a template and cal-
cined at 80 and 600 °C for 4 h, respectively. A broad adsorption peak is seen at 3000–3800 cm1 wavelength range, which is assigned to the stretching modes of O–H bands and related to free water (capillary pore water and surface absorbed water) [19]. The sample calcined at 600 °C has more surface absorbed water and hydroxyl groups than that dried at 80 °C, possibly due to their larger specific surface area. In all samples, the bands at 1000– 1300 cm1 wavelength range are clearly visible, which is attributed to the asymmetric stretching vibrations of Si– O–Si band. The absorption peaks at 800 and 465 cm1 are due to the symmetric stretching vibrations of Si–O–Si band. In addition to these peaks, the peaks at 1640 and 960 cm1 are indicative of the existence of surface silanol groups [20]. On the other hand, it can be seen from Fig. 4(a) that two bands at 2925 and 2850 cm1 are ascribed to C–H stretching modes of the hydrocarbon chain of dodecylamine. The small bands at 1543 and 1460 cm1 are due to the deformation vibration of –CH2– and –CH3 of the incorporated dodecylamine [21]. It should be noted that the peaks related to 2925, 2850, 1640 and 1543 cm1 disappeared at 600 °C, indicating the complete removal of dodecylamine after high temperature
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500
3455 3430
2850 2925
1460 1543
(a)
1090
800 960 465
Transmittance/ a.u.
(b)
1640
Fig. 3. TEM (a) and HRTEM images (b) of silica microspheres obtained using dodecylamine as template and calcined at 600 °C.
1000 1500 2000 2500 3000 3500 4000 -1
Wavenumber /cm
Fig. 4. FTIR spectra of silica microspheres obtained using dodecylamine as template and calcined at 80 °C (a) and 600 °C (b).
treatments. Hence, it could be concluded that the template of dodecylamine was incorporated into the silica microspheres during the growth of the silica microspheres. To examine the influence of the calcination temperature on the surface area and pore structure, the calcination was carried out at 80, 400 and 600 °C. Fig. 5(a) and (b) show that the influence of the calcination temperature on nitrogen adsorption–desorption isotherms and the corresponding BJH pore size distribution curves of the SiO2 powders obtained using hexadecylamine as a template, respectively. All the isotherms of the samples exhibit type IV isotherms without any hysteresis loop. The capillary condensation steps occur at a relative pressure P/P0 of ca. 0.4, which is an indicative of the filling of framework confined mesopores [22]. The steps are slightly shifted to a lower relative pressure, indicating a decrease in framework pore size with increasing calcination temperature. The pore size distributions (PSD) of the samples are determined from desorption branch isotherms via the BJH method (as shown in Fig. 5(b)). The pore size distributions become narrow and pore sizes slightly decrease with increasing calcination temperature (see Table 1). This is due to the fact that organic template is removed from the
Fig. 5. (a) Nitrogen adsorption–desorption isotherms of the SiO2 powders obtained using hexadecylamine as template and calcined at various temperatures (b) corresponding BJH pore size distribution curves.
SiO2 microspheres at high temperature, which causes the densification and shrinkage of SiO2 microspheres. To compare the pore structures of the SiO2 powders synthesized using different templates and calcined at 600 °C, their nitrogen adsorption–desorption isotherms are presented in Fig. 6(a). The isotherm of the C12 sample is type I nitrogen adsorption–desorption isotherm, according to the IUPAC classification [23], characteristic of microporous materials. The isotherm corresponding to
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Table 1 Effects of different template and calcination temperature on BET surface areas and pore parameters of porous SiO2 powders Samples
d100 (nm)
SBETa (m2 g1)
Vpb (mL g1)
Porosityc (%)
Dd HK
BJH
Error levels Dodecylamine (80 °C) Dodecylamine (400 °C) Dodecylamine (600 °C) Hexadecylamine (80 °C) Hexadecylamine (400 °C) Hexadecylamine (600 °C) Octodecylamine (80 °C) Octodecylamine (400 °C) Octodecylamine (600 °C)
±5% 3.5 3.3 3.2 6.0 5.7 5.6 7.6 7.3 7.1
±5% 513 986 1121 297 816 1001 168 641 800
±5% 0.27 0.50 0.54 0.05 0.58 0.65 0.12 0.65 0.79
±5% 42.2 57.5 59.4 11.9 61.1 63.7 24.5 63.7 68.1
±5% 1.50 1.46 1.43
±5%
a b c d e
te (nm)
2.69 2.45 2.44 1.63 3.78 3.81
±5% 2.54 2.35 2.27 4.24 4.13 4.03 7.2 4.7 4.4
BET surface area calculated from the linear part of the BET plot (P/P0 = 0.05–0.25). Total pore volume, taken from the volume of N2 adsorbed at P/P0 = 0.975. The porosity is estimated from the pore volume determined using the adsorption branch of the N2 isotherm curve at the P/P0 = 0.975 single point. Average pore diameter estimated using the branch of the isotherm by the HK or BJH method, respectively. pffiffiadsorption ffi Pore wall thickness is equal to t ¼ 2d 100 = 3 D.
in the pore size with increasing the alkyl chain length of 1-alkylamine. This tendency can also be seen clearly from the corresponding BJH pore size distributions in Fig. 6(b). It shows that the pore sizes increase obviously with increasing the alkyl chain length of 1-alkylamine accompanied by the slightly narrowing of the pore size distributions. The effects of calcination temperature and alkyl chain length of 1-alkylamine on BET specific surface areas and pore parameters of SiO2 powders are summarized in Table 1. It can be seen that the BET specific surface area, porosity and pore volume of the samples obtained using dodecylamine as a template slightly increase with increasing calcination temperatures. The trend is also observed for the C16 and C18 samples. Moreover, it is interesting to note that at 600 °C, with increasing the alkyl chain length of 1-alkylamine, the pore volume and average pore size increase, however, surface area decreases. The pore wall thickness is determined by subtraction of the average pore size from pffiffiffi the pore–pore correlation distance (a0) (a0 ¼ 2d 100 = 3Þ [25]. As the samples are synthesized using the templates with longer alkyl chain, then the pore walls become thicker (as shown in Table 1), which results in a lower BET specific surface area. 4. Discussion
Fig. 6. (a) Nitrogen adsorption–desorption isotherms of the SiO2 powders obtained using different templates and calcined at 600 °C; (b) corresponding BJH pore size distribution curves.
the C16 powders is of type IV isotherm without any hysteresis loop. For the C18 sample, the isotherm of the silica sample is type IV with a H3 type hysteresis loop. This indicates that the powders contain mesopores with narrow slitlike shapes [24]. The capillary condensation steps are shifted to higher relative pressures, indicating an increase
Porous SiO2 microspheres with wormhole-like pore structures could be synthesized using 1-alkylamine as the organic template (S) and TEOS as the inorganic precursor (I). The rapid synthesis of porous silica microspheres may be due to the formation of initial silica nuclei using 1-alkylamine as a basic catalyst, which is like the formation of initial silica nuclei in the Sto¨ber method [26], then the SiO2 particles further grow by the undirectional agglomeration of primary particles, resulting in the formation of submicrometer-sized and micrometer-sized silica spheres. It is well known that the difference between the nucleation
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and growth rates is an important factor for the formation of monodispersed particle [27]. Usually, the distinct separation between nucleation and growth process results in the formation of highly monodispersed particles. Dodecylamine can cause a faster nucleation rate for the formation of silica particles due to its stronger basicity comparing with ammonia, which leads to a great difference between the nucleation and growth rates. Consequently, the monodispersed porous silica microspheres can be obtained using dodecylamine as a hydrolysis catalyst and organic template. Why does the alkyl chain length of 1-alkylamine obviously influence the sizes and morphologies of SiO2 particles? This may be ascribed to the fact that with increasing alkyl chain length of 1-alkylamine, the OH concentration (generated by the hydrolysis of 1-alkylamine) of the solution slightly increases [28]. Therefore, the rate of the hydrolysis of TEOS increases. Also, the rate of hydrolysis relative to the rate of condensation also increases. The stability of small particles decreases as the concentration of OH ions increases, and a dissolution and reprecipitation process occurs, resulting in the formation of larger and un-uniform SiO2 particles [29]. Fig. 2 shows that a shorter alkyl chain for 1-alkylamine is more beneficial to the formation of monodispersed SiO2 spherical particles due to a relative low basicity and steric hindrance. In this neutral templating route, the SI assembly relies on the formation of hydrogen bonding between the amine surfactant (S) and SiO2 oligomer (I) at the micelle interface [30,31]. The SI assembly will proceed due to the generation of the silica oligomer by the hydrolysis of silica precursor. Further crosslinking and polymerization of adjacent silica species result in disordered packing of the micelles and the formation of framework wall. In a mixed solution of water and ethanol, the template first formed a lamellar structure. After addition of TEOS, the lamellar structure is transformed into a wormhole-like structure by the hydrogen bonding interactions between the amine headgroup and the silica oligomer [32]. Of course, a longer alkyl chain template will swell the micelle and lead to the formation of larger pores. The result is good agreement with that described by Igarashi et al [33]. Therefore, the pore size increased from 1.43 to 3.81 nm with increasing the alkyl chain length of template from C12 to C18. The thickness of the pore wall between micelles depends on the degree of condensation of silica, affected by the OH concentration of the solution. When the alkyl chain length of 1-alkylamine increases, then the OH concentration of the solution increases, which leads to a faster condensation reaction between silica-coated micelles. As a result, a thicker pore wall is obtained [25,34]. 5. Conclusion In summary, we have successfully synthesized monodispersed porous silica microspheres with very large specific surface area and disordered pore structures using dodecylamine as a hydrolysis catalyst and organic template. The
silica microspheres were formed probably through a neutral SI assembly mechanism during the hydrolysis and condensation of TEOS. The particle size, morphology and pore structure of silica microspheres depended on the alkyl chain length of 1-alkylamine. The monodispersed silica microspheres could be prepared using dodecylamine as a template. The surface area, porosity and pore volume of silica microspheres increased with increasing calcination temperature. Furthermore, at 600 °C, with increasing the alkyl chain length of 1-alkylamine from C12 to C18, the pore size, porosity and pore volume increased, while, the surface area decreased. This research may provide an easy approach for the control of the morphology and pore structure of silica particles by using 1-alkylamine as a template and hydrolysis catalyst. Acknowledgments This work was supported by the National Natural Science Foundation of China (Project Nos. 50272049 and 20473059), and supported by the Excellent Young Teachers Program of MOE, PR China and the key project of State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (WUT2004Z03). References [1] C.T. Kresege, M.E. Leonowicz, W.J. Roth, J.S. Beck, Nature 359 (1992) 710. [2] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Lenowicz, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [3] M.R. Buchmeiser, J. Chromatogr. A 918 (2001) 233. [4] J.J. Kirkland, F.A. Truszkowski, C.H. Dilks Jr., G.S. Engel, J. Chromatogr. A 890 (2000) 3. [5] L.M. Qi, J.M. Ma, H.M. Cheng, Z.G. Zhao, Chem. Mater. 10 (1998) 1623. [6] S.J. Limmer, T.L. Hubler, G.Z. Cao, J. Sol–Gel Sci. Technol. 15 (1999) 243. [7] M. Etienne, B. Lebeau, A. Walcarius, New J. Chem. 26 (2002) 384. [8] C.E. Fowler, D. Khushalani, S. Mann, Chem. Commun. (2001) 2028. [9] R. Guo, J.G. Yu, L. Zhao, X.J. Zhao, Acta Chim. Sinica 62 (2004) 493. [10] P.G. Ni, P. Dong, B.V. Cheng, X.Y. Li, D.Z. Zhang, Adv. Mater. 13 (2001) 437. [11] R.I. Nooney, D. Thirunavukkarasu, Y.M. Chen, R. Josephs, A.E. Ostafin, Chem. Mater. 14 (2002) 4721. [12] H. Izutsu, F. Mizukami, P.K. Nair, Y. Kiyozumi, K. Maeda, J. Mater. Chem. 5 (1997) 767. [13] R. Vacassy, R.J. Flatt, H. Hofmann, K.S. Choi, R.K. Singh, J. Colloid Interf. Sci. 227 (2000) 302. [14] L.M. Qi, J. Mater. Sci. Lett. 20 (2001) 2153. [15] Y.R. Ma, L.M. Qi, J.M. Ma, Y.Q. Wu, O. Liu, H.M. Cheng, Chem. Mater. 10 (1998) 1623. [16] B. Lefe`vre, A. Galarneau, J. Iapichella, C. Petitto, F. Di Renzo, F. Fajula, Z. Bayram-Hahn, R. Skudas, K. Unger, Chem. Mater. 17 (2005) 601. [17] E.P. Barrett, L.G. Joyner, P.H. Halenda, J. Am. Chem. Soc. 73 (1951) 373. [18] B.D. Yao, L.D. Zhang, J. Mater. Sci. 34 (1999) 5983. [19] X.S. Zhao, G.Q. Lu, J. Phys. Chem. B 102 (1998) 1556. [20] J.G. Yu, J.C. Yu, X.J. Zhao, J. Sol–Gel Sci. Technol. 24 (2002) 95. [21] Y.D. Wang, C.L. Ma, X.D. Sun, H.D. Li, J. Non-Cryst. Solids 319 (2003) 109.
L. Zhao et al. / Journal of Non-Crystalline Solids 351 (2005) 3593–3599 [22] P.T. Tanev, T.J. Pinnavaia, Chem. Mater. 8 (1996) 2068. [23] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603. [24] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic, London, 1982. [25] H.P. Lin, S. Cheng, C.Y. Mou, Microporous Mater. 10 (1997) 111. [26] W. Sto¨ber, A. Fink, E. Bohn, J. Colloid Interf. Sci. 26 (1968) 62. [27] K. Kosuge, P.S. Singh, Chem. Mater. 13 (2001) 2476. [28] K. Kosuge, T. Murakami, N. Kikukawa, M. Takemori, Chem. Mater. 15 (2003) 3184.
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[29] F.J. Arriagada, K. Osseo-Asare, J. Colloid Interf. Sci. 211 (1999) 210. [30] Y.D. Wang, C.L. Ma, X.D. Sun, H.D. Li, Mater. Lett. 54 (2002) 359. [31] C.L. Pan, W.X. Zhang, Y.L. Wang, Z. Zhou, D.Z. Jiang, S.J. Wu, T.H. Wu, Mater. Lett. 57 (2003) 3815. [32] T.R. Pauly, T.J. Pinnavaia, Chem. Mater. 13 (2001) 987. [33] N. Igarashi, K.A. Koyano, Y. Tanaka, S. Nakata, K. Hashimoto, T. Tatsumi, Microporous Mesoporous Mater. 59 (2003) 43. [34] K. Kosuge, P.S. Singh, Microporous Mesoporous Mater. 44 (2001) 139.
A novel approach for the synthesis of monodispersed porous silica microspheres with high surface area Li Zhao a, Jiaguo Yu a
a,*
, Bei Cheng a, Chengzhong Yu
b
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Luoshi Road 122#, Wuhan 430070, PR China b Department of Chemistry, Fudan University, Shanghai 200433, PR China Received 1 February 2005; received in revised form 23 September 2005 Available online 26 October 2005
Abstract Monodispersed porous silica microspheres are synthesized by the hydrolysis and condensation of tetraethoxysilane (TEOS) in a water–ethanol mixed solution containing 1-alkylamine as a template and hydrolysis catalyst. The as-prepared products were characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM), small angle X-ray diffraction (SAXRD), Fourier transform infrared spectroscopy (FTIR) and nitrogen adsorption, respectively. It was found that the alkyl chain length of 1-alkylamine and calcination temperature have an obvious influence on the particle size, morphology, specific surface area and pore structure of the as-prepared silica powder. The specific surface area, porosity and pore volume increased with increasing calcination temperature. Further observation showed that at 600 °C, with increasing the alkyl chain length of template from C12 to C18, the specific surface area decreased and the pore size, porosity and pore volume increased. This research may provide new insight into the control of morphology and pore structures of oxide materials. Ó 2005 Elsevier B.V. All rights reserved. PACS: 61.43.Er; 61.43.Gt; 81.20.Fw
1. Introduction The development of mesoporous silica materials by researchers of Mobil Corporation in 1992 stimulated explosive research on the preparation of porous materials through template approaches [1,2]. That was due to the fact that the porous materials had their potential applications as versatile catalyst, catalyst supports, separation media, and hosts for clusters and nanowires [3–6]. Various types of organic templates including surfactant self-assemblies, block copolymer and polymer lattices have been used to synthesize such porous inorganic materials. Recently, the synthesis of monodispersed spherical particles by the
*
Corresponding author. E-mail address: [email protected] (J. Yu).
0022-3093/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.09.031
sol–gel method has focused more attention on the design of spherical silica-based materials with tailored pore structures and properties [7–10]. In this regard, significant effort has been devoted to the organic templating or molecular imprinting of porous silica systems in which the templates interact covalently or non-covalently with silica precursors [7,11,12]. For example, Vacassy et al. [13] used 3-aminopropyltriethoxy-silane and glycerol as porogens to prepare microporous silica spheres. Qi et al. [14] used double-hydrophilic block copolymers (PEO-b-PMMA) as template under strong acidic conditions to form microporous silica spheres. Ma et al. [15] used a triblock copolymer EO20PO70EO20 as template in combination with a co-surfactant CTAB to prepare silica microspheres via a two-step synthesis process. Lefe`vre et al. [16] reported that preparation of large-pore mesostructured micelle-templated silica spheres. Although a variety of porous silica spherical
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particles have been synthesized so far, almost all the silica spheres have been obtained by using a triphasic system of a quaternary ammonium surfactant with an additional auxiliary organic solvent and co-surfactant. In this study, porous silica microspheres with narrow particle size distribution were successfully prepared by hydrolysis and condensation of tetraethoxysilane (TEOS) in a mixed solution of water, 1-alkylamine and ethanol. The key in this experimental process is the dual role of 1-alkylamine, not only as a porous template but also as a basic catalyst for the hydrolysis and condensation of TEOS. To the best of our knowledge, this was a very simple and more effective method for producing monodispersed silica spherical particles with a very large specific surface area. The formation mechanism of porous silica microspheres with disordered pores was proposed based on these experimental results. 2. Experimental section 2.1. Preparation The porous silica microspheres were synthesized using 1-alkylamine (designated C12 for dodecylamine, C16 for hexadecylamine and C18 for octodecylamine) as a template and hydrolysis catalyst, and TEOS as an inorganic framework sources. In a typical preparation, 0.005 mol of 1-alkylamine was dissolved in a mixed solution of ethanol (160 ml) and distilled water (100 ml). Then, 10 ml TEOS was added dropwise to the above solution at about 15 °C. After 4 h, the white precipitate was filtrated. The products were repeatedly washed with water and ethanol for four times, then dried in a vacuum oven at 80 °C for 4 h, finally calcined at 400 and 600 °C for 4 h in a muffle furnace to remove the templates, respectively. 2.2. Characterization The morphology of the silica products was observed by scanning electron microscopy (SEM) (type JSM-5610LV) with an accelerating voltage of 20 kV. Transmission electron microscopy (TEM) study was carried out on a Philips TECNAL-10 electron microscopy instrument. The sample for TEM was prepared via milling and dispersing the final powders in ethanol. The dispersion was then sonicated and dropped onto carbon-coated copper grids. Small angle X-ray diffraction (XRD) patterns were recorded on an HZG41B-PC X-ray diffractometer using Cu-Ka radiation with a 2h range of 1–5°. Fourier transform infrared (FTIR) spectra on pellets of the samples mixed with KBr were recorded on a Nicolet Magna 8100 FTIR spectrometer. The Brunauer–Emmett–Teller (BET) surface area (SBET) and pore parameters of the silica products were analyzed by nitrogen adsorption–desorption isotherm measurements on an AUTOSORB-1 (Quantachrome Instruments, USA) nitrogen adsorption apparatus. For the samples dried at 80 °C, they were degassed at 70 °C prior to actual measure-
ments. However, for the samples calcined at high temperatures (from 400 to 600 °C), the degassing temperature was 180 °C. The BET surface area was determined by the multipoint BET method using the adsorption data in the relative pressure (p/p0) range of 0.05–0.25. The desorption branch of the nitrogen isotherm was used to determine the pore size distribution using Barret, Joyner, and Halender (BJH) method [17]. The porosity was calculated according to the following equations (the skeleton specific volume of SiO2 is taken as 0.37 cm3 g1) [18], P ¼ V p =ðV p þ 0:37Þ;
ð1Þ
3
V p ¼ 1:547 10 V d ;
ð2Þ
where Vp is the volume of the liquidated nitrogen corresponding to the total pore volume, which was calculated from the saturation adsorption volume at STP, Vd. 3. Results Fig. 1 shows the small angle XRD patterns of the assynthesized products using dodecylamine, hexadecylamine and octodecylamine as templates and calcined at 600 °C for 4 h. All XRD patterns are similar and exhibit a broad low-angle diffraction peak corresponding to d100 spacing of 3.2, 5.6, 7.1 nm for C12, C16 and C18, respectively. It is found that the d100 spacing of the silica samples increases with increasing the alkyl chain length of template. This is attributed to the pore size expansion as the alkyl chain length increased from C12 to C18. The absence of higher angle refraction peaks indicates that these particles lack the long-range order in the pore arrangement that was commonly seen for conventionally prepared highly ordered microporous and mesoporous silica materials. Fig. 2 shows the influence of different templates on the particle sizes and morphologies of SiO2 particles calcined at 80 and 600 °C. It can be seen that the size and morphology of SiO2 particles are obviously different for different templates. Furthermore, the morphologies of the as-syn-
(c) (b) (a)
Fig. 1. Small angle XRD patterns of porous silica microspheres obtained using dodecylamine (a), hexadecylamine (b), and octodecylamine (c) as templates and calcined at 600 °C for 4 h.
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Fig. 2. SEM images of silica microspheres obtained using dodecylamine as template and calcined at 80 °C (a) and 600 °C (b), hexadecylamine as template and calcined at 80 °C (c) and 600 °C (d), and octodecylamine as template and calcined at 80 °C (e) and 600 °C (f).
thesized particle are retained after the calcination treatments. This result shows that the calcination treatments cannot obviously affect the size and morphology of silica particles. For C12 powder, the SiO2 particles show uniform size and spherical morphology. The average diameter of the SiO2 microspheres is about 700 nm. For C16 powder, the SiO2 particles appear polydispersity. And the average size of the SiO2 particles obviously becomes large and is about 1.5 lm. In contrast, the C18 powders have a largest particle size and exhibit some plate-like aggregate particles in addition to some spherical particles. Therefore, the morphologies and sizes of SiO2 particles are strongly influenced by the alkyl chain length of 1-alkylamine. The particle size, monodispersity and morphology of SiO2 particles can be clearly observed in the TEM image. Fig. 3(a) shows TEM images of the monodispersed silica microspheres synthesized using dodecylamine as a template. As can be seen from HRTEM image (as shown in Fig. 3(b)), wormhole-like pore structures are randomly connected and lack discernible long-range order in the pore arrangement among the small SiO2 particles. Fig. 4(a) and (b) show the FTIR spectra of silica microspheres obtained using dodecylamine as a template and cal-
cined at 80 and 600 °C for 4 h, respectively. A broad adsorption peak is seen at 3000–3800 cm1 wavelength range, which is assigned to the stretching modes of O–H bands and related to free water (capillary pore water and surface absorbed water) [19]. The sample calcined at 600 °C has more surface absorbed water and hydroxyl groups than that dried at 80 °C, possibly due to their larger specific surface area. In all samples, the bands at 1000– 1300 cm1 wavelength range are clearly visible, which is attributed to the asymmetric stretching vibrations of Si– O–Si band. The absorption peaks at 800 and 465 cm1 are due to the symmetric stretching vibrations of Si–O–Si band. In addition to these peaks, the peaks at 1640 and 960 cm1 are indicative of the existence of surface silanol groups [20]. On the other hand, it can be seen from Fig. 4(a) that two bands at 2925 and 2850 cm1 are ascribed to C–H stretching modes of the hydrocarbon chain of dodecylamine. The small bands at 1543 and 1460 cm1 are due to the deformation vibration of –CH2– and –CH3 of the incorporated dodecylamine [21]. It should be noted that the peaks related to 2925, 2850, 1640 and 1543 cm1 disappeared at 600 °C, indicating the complete removal of dodecylamine after high temperature
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500
3455 3430
2850 2925
1460 1543
(a)
1090
800 960 465
Transmittance/ a.u.
(b)
1640
Fig. 3. TEM (a) and HRTEM images (b) of silica microspheres obtained using dodecylamine as template and calcined at 600 °C.
1000 1500 2000 2500 3000 3500 4000 -1
Wavenumber /cm
Fig. 4. FTIR spectra of silica microspheres obtained using dodecylamine as template and calcined at 80 °C (a) and 600 °C (b).
treatments. Hence, it could be concluded that the template of dodecylamine was incorporated into the silica microspheres during the growth of the silica microspheres. To examine the influence of the calcination temperature on the surface area and pore structure, the calcination was carried out at 80, 400 and 600 °C. Fig. 5(a) and (b) show that the influence of the calcination temperature on nitrogen adsorption–desorption isotherms and the corresponding BJH pore size distribution curves of the SiO2 powders obtained using hexadecylamine as a template, respectively. All the isotherms of the samples exhibit type IV isotherms without any hysteresis loop. The capillary condensation steps occur at a relative pressure P/P0 of ca. 0.4, which is an indicative of the filling of framework confined mesopores [22]. The steps are slightly shifted to a lower relative pressure, indicating a decrease in framework pore size with increasing calcination temperature. The pore size distributions (PSD) of the samples are determined from desorption branch isotherms via the BJH method (as shown in Fig. 5(b)). The pore size distributions become narrow and pore sizes slightly decrease with increasing calcination temperature (see Table 1). This is due to the fact that organic template is removed from the
Fig. 5. (a) Nitrogen adsorption–desorption isotherms of the SiO2 powders obtained using hexadecylamine as template and calcined at various temperatures (b) corresponding BJH pore size distribution curves.
SiO2 microspheres at high temperature, which causes the densification and shrinkage of SiO2 microspheres. To compare the pore structures of the SiO2 powders synthesized using different templates and calcined at 600 °C, their nitrogen adsorption–desorption isotherms are presented in Fig. 6(a). The isotherm of the C12 sample is type I nitrogen adsorption–desorption isotherm, according to the IUPAC classification [23], characteristic of microporous materials. The isotherm corresponding to
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Table 1 Effects of different template and calcination temperature on BET surface areas and pore parameters of porous SiO2 powders Samples
d100 (nm)
SBETa (m2 g1)
Vpb (mL g1)
Porosityc (%)
Dd HK
BJH
Error levels Dodecylamine (80 °C) Dodecylamine (400 °C) Dodecylamine (600 °C) Hexadecylamine (80 °C) Hexadecylamine (400 °C) Hexadecylamine (600 °C) Octodecylamine (80 °C) Octodecylamine (400 °C) Octodecylamine (600 °C)
±5% 3.5 3.3 3.2 6.0 5.7 5.6 7.6 7.3 7.1
±5% 513 986 1121 297 816 1001 168 641 800
±5% 0.27 0.50 0.54 0.05 0.58 0.65 0.12 0.65 0.79
±5% 42.2 57.5 59.4 11.9 61.1 63.7 24.5 63.7 68.1
±5% 1.50 1.46 1.43
±5%
a b c d e
te (nm)
2.69 2.45 2.44 1.63 3.78 3.81
±5% 2.54 2.35 2.27 4.24 4.13 4.03 7.2 4.7 4.4
BET surface area calculated from the linear part of the BET plot (P/P0 = 0.05–0.25). Total pore volume, taken from the volume of N2 adsorbed at P/P0 = 0.975. The porosity is estimated from the pore volume determined using the adsorption branch of the N2 isotherm curve at the P/P0 = 0.975 single point. Average pore diameter estimated using the branch of the isotherm by the HK or BJH method, respectively. pffiffiadsorption ffi Pore wall thickness is equal to t ¼ 2d 100 = 3 D.
in the pore size with increasing the alkyl chain length of 1-alkylamine. This tendency can also be seen clearly from the corresponding BJH pore size distributions in Fig. 6(b). It shows that the pore sizes increase obviously with increasing the alkyl chain length of 1-alkylamine accompanied by the slightly narrowing of the pore size distributions. The effects of calcination temperature and alkyl chain length of 1-alkylamine on BET specific surface areas and pore parameters of SiO2 powders are summarized in Table 1. It can be seen that the BET specific surface area, porosity and pore volume of the samples obtained using dodecylamine as a template slightly increase with increasing calcination temperatures. The trend is also observed for the C16 and C18 samples. Moreover, it is interesting to note that at 600 °C, with increasing the alkyl chain length of 1-alkylamine, the pore volume and average pore size increase, however, surface area decreases. The pore wall thickness is determined by subtraction of the average pore size from pffiffiffi the pore–pore correlation distance (a0) (a0 ¼ 2d 100 = 3Þ [25]. As the samples are synthesized using the templates with longer alkyl chain, then the pore walls become thicker (as shown in Table 1), which results in a lower BET specific surface area. 4. Discussion
Fig. 6. (a) Nitrogen adsorption–desorption isotherms of the SiO2 powders obtained using different templates and calcined at 600 °C; (b) corresponding BJH pore size distribution curves.
the C16 powders is of type IV isotherm without any hysteresis loop. For the C18 sample, the isotherm of the silica sample is type IV with a H3 type hysteresis loop. This indicates that the powders contain mesopores with narrow slitlike shapes [24]. The capillary condensation steps are shifted to higher relative pressures, indicating an increase
Porous SiO2 microspheres with wormhole-like pore structures could be synthesized using 1-alkylamine as the organic template (S) and TEOS as the inorganic precursor (I). The rapid synthesis of porous silica microspheres may be due to the formation of initial silica nuclei using 1-alkylamine as a basic catalyst, which is like the formation of initial silica nuclei in the Sto¨ber method [26], then the SiO2 particles further grow by the undirectional agglomeration of primary particles, resulting in the formation of submicrometer-sized and micrometer-sized silica spheres. It is well known that the difference between the nucleation
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and growth rates is an important factor for the formation of monodispersed particle [27]. Usually, the distinct separation between nucleation and growth process results in the formation of highly monodispersed particles. Dodecylamine can cause a faster nucleation rate for the formation of silica particles due to its stronger basicity comparing with ammonia, which leads to a great difference between the nucleation and growth rates. Consequently, the monodispersed porous silica microspheres can be obtained using dodecylamine as a hydrolysis catalyst and organic template. Why does the alkyl chain length of 1-alkylamine obviously influence the sizes and morphologies of SiO2 particles? This may be ascribed to the fact that with increasing alkyl chain length of 1-alkylamine, the OH concentration (generated by the hydrolysis of 1-alkylamine) of the solution slightly increases [28]. Therefore, the rate of the hydrolysis of TEOS increases. Also, the rate of hydrolysis relative to the rate of condensation also increases. The stability of small particles decreases as the concentration of OH ions increases, and a dissolution and reprecipitation process occurs, resulting in the formation of larger and un-uniform SiO2 particles [29]. Fig. 2 shows that a shorter alkyl chain for 1-alkylamine is more beneficial to the formation of monodispersed SiO2 spherical particles due to a relative low basicity and steric hindrance. In this neutral templating route, the SI assembly relies on the formation of hydrogen bonding between the amine surfactant (S) and SiO2 oligomer (I) at the micelle interface [30,31]. The SI assembly will proceed due to the generation of the silica oligomer by the hydrolysis of silica precursor. Further crosslinking and polymerization of adjacent silica species result in disordered packing of the micelles and the formation of framework wall. In a mixed solution of water and ethanol, the template first formed a lamellar structure. After addition of TEOS, the lamellar structure is transformed into a wormhole-like structure by the hydrogen bonding interactions between the amine headgroup and the silica oligomer [32]. Of course, a longer alkyl chain template will swell the micelle and lead to the formation of larger pores. The result is good agreement with that described by Igarashi et al [33]. Therefore, the pore size increased from 1.43 to 3.81 nm with increasing the alkyl chain length of template from C12 to C18. The thickness of the pore wall between micelles depends on the degree of condensation of silica, affected by the OH concentration of the solution. When the alkyl chain length of 1-alkylamine increases, then the OH concentration of the solution increases, which leads to a faster condensation reaction between silica-coated micelles. As a result, a thicker pore wall is obtained [25,34]. 5. Conclusion In summary, we have successfully synthesized monodispersed porous silica microspheres with very large specific surface area and disordered pore structures using dodecylamine as a hydrolysis catalyst and organic template. The
silica microspheres were formed probably through a neutral SI assembly mechanism during the hydrolysis and condensation of TEOS. The particle size, morphology and pore structure of silica microspheres depended on the alkyl chain length of 1-alkylamine. The monodispersed silica microspheres could be prepared using dodecylamine as a template. The surface area, porosity and pore volume of silica microspheres increased with increasing calcination temperature. Furthermore, at 600 °C, with increasing the alkyl chain length of 1-alkylamine from C12 to C18, the pore size, porosity and pore volume increased, while, the surface area decreased. This research may provide an easy approach for the control of the morphology and pore structure of silica particles by using 1-alkylamine as a template and hydrolysis catalyst. Acknowledgments This work was supported by the National Natural Science Foundation of China (Project Nos. 50272049 and 20473059), and supported by the Excellent Young Teachers Program of MOE, PR China and the key project of State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (WUT2004Z03). References [1] C.T. Kresege, M.E. Leonowicz, W.J. Roth, J.S. Beck, Nature 359 (1992) 710. [2] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Lenowicz, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [3] M.R. Buchmeiser, J. Chromatogr. A 918 (2001) 233. [4] J.J. Kirkland, F.A. Truszkowski, C.H. Dilks Jr., G.S. Engel, J. Chromatogr. A 890 (2000) 3. [5] L.M. Qi, J.M. Ma, H.M. Cheng, Z.G. Zhao, Chem. Mater. 10 (1998) 1623. [6] S.J. Limmer, T.L. Hubler, G.Z. Cao, J. Sol–Gel Sci. Technol. 15 (1999) 243. [7] M. Etienne, B. Lebeau, A. Walcarius, New J. Chem. 26 (2002) 384. [8] C.E. Fowler, D. Khushalani, S. Mann, Chem. Commun. (2001) 2028. [9] R. Guo, J.G. Yu, L. Zhao, X.J. Zhao, Acta Chim. Sinica 62 (2004) 493. [10] P.G. Ni, P. Dong, B.V. Cheng, X.Y. Li, D.Z. Zhang, Adv. Mater. 13 (2001) 437. [11] R.I. Nooney, D. Thirunavukkarasu, Y.M. Chen, R. Josephs, A.E. Ostafin, Chem. Mater. 14 (2002) 4721. [12] H. Izutsu, F. Mizukami, P.K. Nair, Y. Kiyozumi, K. Maeda, J. Mater. Chem. 5 (1997) 767. [13] R. Vacassy, R.J. Flatt, H. Hofmann, K.S. Choi, R.K. Singh, J. Colloid Interf. Sci. 227 (2000) 302. [14] L.M. Qi, J. Mater. Sci. Lett. 20 (2001) 2153. [15] Y.R. Ma, L.M. Qi, J.M. Ma, Y.Q. Wu, O. Liu, H.M. Cheng, Chem. Mater. 10 (1998) 1623. [16] B. Lefe`vre, A. Galarneau, J. Iapichella, C. Petitto, F. Di Renzo, F. Fajula, Z. Bayram-Hahn, R. Skudas, K. Unger, Chem. Mater. 17 (2005) 601. [17] E.P. Barrett, L.G. Joyner, P.H. Halenda, J. Am. Chem. Soc. 73 (1951) 373. [18] B.D. Yao, L.D. Zhang, J. Mater. Sci. 34 (1999) 5983. [19] X.S. Zhao, G.Q. Lu, J. Phys. Chem. B 102 (1998) 1556. [20] J.G. Yu, J.C. Yu, X.J. Zhao, J. Sol–Gel Sci. Technol. 24 (2002) 95. [21] Y.D. Wang, C.L. Ma, X.D. Sun, H.D. Li, J. Non-Cryst. Solids 319 (2003) 109.
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